Behavioral and biochemical effect of chronic treatment with D-1 and/or D-2 dopamine agonists in MPTP monkeys

The Management of Parkinson's Disease: An Overview of the Current Advancements in Drug Delivery Systems

Parkinson's disease (PD) has significantly affected a large proportion of the elderly population worldwide. According to the World Health Organization, approximately 8.5 million people worldwide are living with PD. In the United States, an estimated one million people are living with PD, with approximately 60,000 new cases diagnosed every year. Conventional therapies available for Parkinson's disease are associated with limitations such as the wearing-off effect, on-off period, episodes of motor freezing, and dyskinesia. In this review, a comprehensive overview of the latest advances in DDSs used to reduce the limitations of current therapies will be presented, and both their promising features and drawbacks will be discussed. We are also particularly interested in the technical properties, mechanism, and release patterns of incorporated drugs, as well as nanoscale delivery strategies to overcome the blood-brain barrier.

Non-human primate models of PD to test novel therapies

Non-human primate (NHP) models of Parkinson disease show many similarities with the human disease. They are very useful to test novel pharmacotherapies as reviewed here. The various NHP models of this disease are described with their characteristics including the macaque, the marmoset, and the squirrel monkey models. Lesion-induced and genetic models are described. There is no drug to slow, delay, stop, or cure Parkinson disease; available treatments are symptomatic. The dopamine precursor, L-3,4-dihydroxyphenylalanine (L-Dopa) still remains the gold standard symptomatic treatment of Parkinson. However, involuntary movements termed L-Dopa-induced dyskinesias appear in most patients after chronic treatment and may become disabling. Dyskinesias are very difficult to manage and there is only amantadine approved providing only a modest benefit. In this respect, NHP models have been useful to seek new drug targets, since they reproduce motor complications observed in parkinsonian patients. Therapies to treat motor symptoms in NHP models are reviewed with a discussion of their translational value to humans. Disease-modifying treatments tested in NHP are reviewed as well as surgical treatments. Many biochemical changes in the brain of post-mortem Parkinson disease patients with dyskinesias are reviewed and compare well with those observed in NHP models. Non-motor symptoms can be categorized into psychiatric, autonomic, and sensory symptoms. These symptoms are present in most parkinsonian patients and are already installed many years before the pre-motor phase of the disease. The translational usefulness of NHP models of Parkinson is discussed for non-motor symptoms.

mGlu5, Dopamine D2 and Adenosine A2A Receptors in L-DOPA-induced Dyskinesias

Patients with Parkinson's disease (PD) receiving L-3,4-dihydroxyphenylalanine (L-DOPA, the gold-standard treatment for this disease) frequently develop abnormal involuntary movements, termed L-DOPA-induced dyskinesias (LID). Glutamate overactivity is well documented in PD and LID. An approach to manage LID is to add to L-DOPA specific agents to reduce dyskinesias such as metabotropic glutamate receptor (mGlu receptor) drugs. This article reviews the contribution of mGlu type 5 (mGlu5) receptors in animal models of PD. Several mGlu5 negative allosteric modulators acutely attenuate LID in 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP) monkeys and 6-hydroxydopamine(6-OHDA)-lesioned rats. Chronic administration of mGlu5 negative allosteric modulators to MPTP monkeys and 6-OHDA rats also attenuates LID while maintaining the antiparkinsonian effect of L-DOPA. Radioligand autoradiography shows an elevation of striatal mGlu5 receptors of dyskinetic L-DOPA-treated MPTP monkeys but not in those without LID. The brain molecular correlates of the long-term effect of mGlu5 negative allosteric modulators treatments with L-DOPA attenuating development of LID was shown to extend beyond mGlu5 receptors with normalization of glutamate activity in the basal ganglia of L-DOPA-induced changes of NMDA, AMPA, mGlu2/3 receptors and VGlut2 transporter. In the basal ganglia, mGlu5 receptor negative allosteric modulators also normalize the L-DOPA-induced changes of dopamine D2receptors, their associated signaling proteins (ERK1/2 and Akt/GSK3β) and neuropeptides (preproenkephalin, preprodynorphin) as well as the adenosine A2A receptors expression. These results show in animal models of PD reduction of LID with mGlu5 negative allosteric modulation associated with normalization of glutamate, dopamine and adenosine receptors suggesting a functional link of these receptors in chronic treatment with L-DOPA.

Levodopa-Induced Dyskinesia: Facts and Fancy. What Does the MPTP Monkey Model Tell Us?

Levodopa-induced dyskinesia, one of the most frequent long-term side effects of antiparkinsonian therapy, is often attributed to denervation supersensitivity of dopamine receptors and perhaps more specifically the D-1 receptor. The available evidence based not only on clinico-pathological studies in patients but also on results of experiments performed on methyl-phenyl-tetrahydropyridine (MPTP)-treated monkeys suggests that the mechanisms may be more complex than heretofore believed. Thus it appears that no single receptor is the sole culprit, that some form of denervation supersensitivity is probably involved but not in the form of increased density of dopamine receptors. Moreover, other neurotransmitter systems must be considered such as GABA, excitatory aminoacids and peptides. The MPTP monkey model remains very useful for predicting the potential of new drugs for inducing dyskinesia. Such trials however must be performed in drug-naive animals.

Pharmacological Treatments Inhibiting Levodopa-Induced Dyskinesias in MPTP-Lesioned Monkeys: Brain Glutamate Biochemical Correlates

Anti-glutamatergic drugs can relieve Parkinson’s disease (PD) symptoms and decrease l-3,4-dihydroxyphenylalanine (l-DOPA)-induced dyskinesias (LID). This review reports relevant studies investigating glutamate receptor subtypes in relation to motor complications in PD patients and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys. Antagonists of the ionotropic glutamate receptors, such as N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, display antidyskinetic activity in PD patients and animal models such as the MPTP monkey. Metabotropic glutamate 5 (mGlu5) receptor antagonists were shown to reduce the severity of LID in PD patients as well as in already dyskinetic non-human primates and to prevent the development of LID in de novo treatments in non-human primates. An increase in striatal post-synaptic NMDA, AMPA, and mGlu5 receptors is documented in PD patients and MPTP monkeys with LID. This increase can be prevented in MPTP monkeys with the addition of a specific glutamate receptor antagonist to the l-DOPA treatment and also with drugs of various pharmacological specificities suggesting multiple receptor interactions. This is yet to be well documented for presynaptic mGlu4 and mGlu2/3 and offers additional new promising avenues.

Long-term treatment with l-DOPA and an mGlu5 receptor antagonist prevents changes in brain basal ganglia dopamine receptors, their associated signaling proteins and neuropeptides in parkinsonian monkeys

Brain glutamate overactivity is well documented in Parkinson's disease (PD) and antiglutamatergic drugs decrease L-3,4-dihydroxyphenylalanine (l-DOPA)-induced dyskinesias (LID); the implication of dopamine neurotransmission is not documented in this anti-LID activity. Therefore, we evaluated changes of dopamine receptors, their associated signaling proteins and neuropeptides mRNA, in normal control monkeys, in saline-treated 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys and in L-DOPA-treated MPTP monkeys, without or with an adjunct treatment to reduce the development of LID: 2-methyl-6-(phenylethynyl)pyridine (MPEP), the prototypal metabotropic glutamate 5 (mGlu5) receptor antagonist. All de novo treatments were administered for 1 month and the animals were sacrificed thereafter. MPTP monkeys treated with l-DOPA + MPEP developed significantly less LID than MPTP monkeys treated with l-DOPA alone. [(3)H]SCH-23390 specific binding to D1 receptors of all MPTP monkeys was decreased as compared to controls in the basal ganglia and no difference was observed between all MPTP groups, while striatal D1 receptor mRNA levels remained unchanged. [(3)H]raclopride specific binding to striatal D2 receptors and mRNA levels of D2 receptors were increased in MPTP monkeys compared to controls; l-DOPA treatment reduced this binding in MPTP monkeys while it remained elevated with the l-DOPA + MPEP treatment. Striatal [(3)H]raclopride specific binding correlated positively with D2 receptor mRNA levels of all MPTP-lesioned monkeys. Striatal preproenkephalin/preprodynorphin mRNA levels and phosphorylated ERK1/2 and Akt/GSK3β levels increased only in L-DOPA-treated MPTP monkeys as compared to controls, saline treated-MPTP and l-DOPA + MPEP treated MPTP monkeys. Hence, reduction of development of LID with MPEP was associated with changes in D2 receptors, their associated signaling proteins and neuropeptides.

Continuous dopaminergic stimulation: clinical aspects and experimental bases

In patients with Parkinson disease, pulsatile administration of dopaminergic drugs is associated with motor fluctuations and dyskinesias. By contrast, treatments that provide more continuous dopaminergic stimulation are associated with less intense motor complications. This can be achieved by using drugs with longer half-lives, delayed release formulations, and routes of administration that permit continuous delivery. The mechanisms by which different modes of dopaminergic treatment (pulsatile or continuous) determine the motor response are not fully understood. However, the use of experimental models of parkinsonism has helped understand the motor complications associated with pulsatile dopamine replacement. These studies have provided important insights into the biochemical and molecular changes in the basal ganglia in response to continuous stimulation. In addition, these models have facilitated the development of new treatments that may stabilize the motor response and the biochemical alterations in the basal ganglia to provide more efficient forms of continuous dopaminergic stimulation in patients with Parkinson disease.

Translation of nondopaminergic treatments for levodopa-induced dyskinesia from MPTP-lesioned nonhuman primates to phase IIa clinical studies: keys to success and roads to failure

Studies in MPTP-lesioned nonhuman primates have demonstrated the potential of nondopaminergic drugs in reducing the problems of levodopa-induced dyskinesia (LID). Here we review the process of translating findings from the monkey to man. Agents targeting glutamate, adensosine, noradrenaline, 5-hydroxytryptamine, cannabinoid, and opioid transmitter systems have been assessed for antidyskinetic potential in human studies. Eleven nondopaminergic drugs with antidyskinetic efficacy in the MPTP primate have been advanced to proof-of-concept phase IIa trials in PD patients (amantadine, istradefylline, idazoxan, fipamezole, sarizotan, quetiapine, clozapine, nabilone, rimonabant, naloxone, and naltrexone). For all six nondopaminergic transmitter systems reviewed, the MPTP-lesioned primate correctly predicted phase II efficacy of at least one drug. Of the 11 specific molecules tested in both monkeys and humans, 8 showed clear antidyskinetic properties in both human and monkey. In the instances where the primate studies did not, or did not consistently, predict the outcome of the human studies, the discrepancy may reflect limitations in the validity of the model or limitations in the design of either the clinical or the preclinical studies. We find that the major determinant of success in predicting efficacy is to ensure that primate studies are conducted in a statistically rigorous way and incorporate designs and outcome measures with clinical applicability. On the other hand, phase IIa trials should strive to replicate the preclinical study, especially in terms of protocol, drug dose equivalence, and outcome measure, so as to test the same hypothesis. Failure to meet these criteria carries the risk of false negative conclusions in phase IIa trials.

Validation of the l-dopa-induced dyskinesia in the 6-OHDA model and evaluation of the effects of selective dopamine receptor agonists and antagonists

Current treatments for Parkinson's disease (PD) rely on a dopamine replacement strategy and are reasonably effective, particularly in the early stages of the disease. However, chronic dopaminergic therapy is limited by the development of a range of side effects, including dyskinesia. This has led to a search for alternative treatments. Transplantation of foetal nigral dopamine neurons is a rational approach and many studies have shown that it can improve motor functions in parkinsonian rodents, primates and man. Recently, however, two clinical trials have reported an exacerbation of dyskinesias in some transplanted patients, raising concerns about the safety of the transplantation strategy. To study this issue, we have reproduced the l-dopa-induced dyskinesia model developed by Cenci et al. [M.A. Cenci, C.S. Lee, A. Bjorklund, l-DOPA-induced dyskinesia in the rat is associated with striatal overexpression of prodynorphin- and glutamic acid decarboxylase mRNA, Eur. J. Neurosci. 10 (1998) 2694-2706] in the rat. We find that their abnormal involuntary movements rating scale is easy to apply and consistent to use. Moreover, the Schallert forelimb placing test has been used to assess l-dopa-induced recovery of function and we find that the rats continue to show good recovery on this test, even while they are exhibiting abnormal dyskinetic side effects. To further evaluate this model, we have studied the effects of selective dopamine receptor antagonists and agonists for D1, D2 and D3 receptors. Antagonists of all three receptors are able to block the l-dopa-induced dyskinesia without interfering with the beneficial effects of l-dopa on the placing test. This indicates that the effects of chronic l-dopa on recovery of parkinsonian symptoms and on induction of dyskinetic side effects can be dissociated, which may provide the basis for developing novel combination treatments, e.g. using grafts while blocking the unwanted adverse effects of the drugs.

Relevance of the MPTP primate model in the study of dyskinesia priming mechanisms

For nearly 20 years, the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) primate model has allowed great strides to be made in our understanding of the maladaptive changes underlying the levodopa-related motor response complications occurring in most parkinsonian patients. Studies indicate that sustained dopamine D2 receptor occupancy can prevent and reverse existing dyskinesias. Recent experiments in levodopa-treated MPTP animals, co-administered either a threshold dose of cabergoline or a glutamate NMDA NR2B-selective antagonist (CI-1041), have afforded protection against dyskinesia, perhaps through presynaptic inhibition of glutamate release and blockade of supersensitive postsynaptic NMDA receptors in the striatum, respectively. Some of the biochemical events that have correlated with dyskinesias, namely upregulated GABA(A) receptors in the internal pallidum, rise in pre-proenkephalin-A gene expression in the striatum, and upregulated striatal glutamate ionotropic receptors and adenosine A(2a) receptors, may be counteracted by these preventive strategies.

Behavioral changes are not directly related to striatal monoamine levels, number of nigral neurons, or dose of parkinsonian toxin MPTP in mice

Behavioral analyses of mice intoxicated by the parkinsonian toxin 1-methyl-4-phenyl-1,2,3,6,-tetrahydropyridine (MPTP) have generated conflicting results. We therefore analyzed the relationship between behavioral changes, loss of monoamine levels, and loss of dopaminergic cell bodies in groups of mice intoxicated with acute or subchronic MPTP protocols. Despite a higher degree of neuronal loss in the mice intoxicated using subchronic protocols, dopamine loss was severe and homogeneous in the striatum in all groups. Dopamine levels were less severely reduced in the frontal cortex in the three groups of MPTP-intoxicated mice. Norepinephrine and serotonin levels in the striatum were decreased only in the mice intoxicated with the acute protocol. The most surprising result was that the mice intoxicated with the subchronic protocols were more active than the saline-treated mice. As reported in rats with dopamine depletion in the prefrontal cortex, the hyperactivity observed in our mice could be due to the reduced dopamine levels detected in this structure.

Effect of pulsatile administration of levodopa on dyskinesia induction in drug-naïve MPTP-treated common marmosets: effect of dose, frequency of administration, and brain exposure

Levodopa (L-dopa) consistently primes basal ganglia for the appearance of dyskinesia in parkinsonian patients and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride (MPTP) -treated primates. This finding may reflect its relatively short duration of effects resulting in pulsatile stimulation of postsynaptic dopamine receptors in the striatum. We have compared the relationship between L-dopa dose and frequency of administration on dyskinesia initiation in drug-naïve, MPTP-treated common marmosets. We have also studied the effect of increased brain exposure to pulsatile administration by combining a low-dose of L-dopa with the peripheral catechol-O-methyltransferase inhibitor (COMT-I), entacapone. Pulsatile administration of a low (dose range, 5.0-7.5 mg/kg p.o.) or a high (12.5 mg/kg) dose of L-dopa plus carbidopa b.i.d. produced a dose-related reversal of motor deficits. Repeated administration of low and high doses of L-dopa for 26 days to drug-naïve, MPTP-treated animals also caused a dose-related induction of peak-dose dyskinesia. Repeated administration of high-dose L-dopa b.i.d. compared to once daily caused a frequency-related improvement of motor symptoms, resulting in a more rapid and initially more intense appearance of peak-dose dyskinesia. Administration of low-dose L-dopa b.i.d. for 26 days in combination with entacapone enhanced the increase in locomotor activity and reversal of disability produced by L-dopa alone, but with no obvious change in duration of L-dopa's effect. However, combining entacapone with L-dopa resulted in the more rapid appearance of dyskinesia, which was initially more severe than occurred with L-dopa alone. Importantly, increasing pulsatile exposure of brain to L-dopa by preventing its peripheral breakdown also increases dyskinesia induction.

The contribution of the MPTP-treated primate model to the development of new treatment strategies for Parkinson's disease

Current research into Parkinson's disease (PD) is directed at developing novel agents and strategies for improved symptomatic management. The aim of this research is to provide effective and maintained symptom control throughout the course of the disease without loss of efficacy and without priming the basal ganglia for the onset of dyskinesia. To achieve these objectives, it is important to have relevant animal models of PD in which new pharmacological agents and treatment strategies can be assessed prior to clinical assessment. At present, the most effective experimental model of PD is the methyl phenyl tetrahydropyridine (MPTP)-treated primate. Primates treated with MPTP develop motor disturbances resembling those seen in idiopathic PD, including bradykinesia, rigidity and postural abnormalities. In addition, MPTP-treated primates are responsive to all commonly used antiparkinsonian agents and display treatment-associated motor complications such as dyskinesia, wearing-off and on-off, which occur during the long-term treatment of the illness. This review examines how studies conducted in MPTP-treated primates have contributed to the development of dopaminergic therapies. There is now accumulating evidence that the pulsatile manner in which short-acting agents stimulate striatal dopamine receptors is a key contributing factor to the priming of the basal ganglia for dyskinesia induction. It has been suggested that providing more continuous stimulation of dopamine receptors will avoid the development of motor complications, particularly dyskinesia. So far, the actions of all commonly used antiparkinsonian drugs assessed in MPTP-treated primates have proved to be highly predictive of drug action in PD. These primate studies have demonstrated that long-acting dopamine agonists and levodopa given in combination with a catechol-O-methyl transferase (COMT) inhibitor (to increase its relatively short half-life), induce significantly less dyskinesia than occurs with standard levodopa therapy.

Both short- and long-acting D-1/D-2 dopamine agonists induce less dyskinesia than L-DOPA in the MPTP-lesioned common marmoset (Callithrix jacchus)

The current concept of dyskinesia is that pulsatile stimulation of D-1 or D-2 receptors by L-DOPA or short-acting dopamine agonists is more likely to induce dyskinesia compared to long-acting drugs producing more continuous receptor stimulation. We now investigate the ability of two mixed D-1/D-2 agonists, namely pergolide (long-acting) and apomorphine (short-acting), to induce dyskinesia in drug-nai;ve MPTP-lesioned primates, compared to L-DOPA. Adult common marmosets (Callithrix jacchus) were lesioned with MPTP (2 mg/kg/day sc for 5 days) and subsequently treated with equieffective antiparkinsonian doses of L-DOPA, apomorphine, or pergolide for 28 days. L-DOPA, apomorphine, and pergolide reversed the MPTP-induced motor deficits to the same degree with no difference in peak response. L-DOPA and apomorphine had a rapid onset of action and short duration of effect producing a pulsatile motor response, while pergolide had a slow onset and long-lasting activity producing a continuous profile of motor stimulation. L-DOPA rapidly induced dyskinesia that increased markedly in severity and frequency over the course of the study, impairing normal motor activity by day 20. Dyskinesia in animals treated with pergolide or apomorphine increased steadily, reaching mild to moderate severity but remaining significantly less marked than that produced by L-DOPA. There was no difference in the intensity of dyskinesia produced by apomorphine and pergolide. These data suggest that factors other than duration of drug action may be important in the induction of dyskinesia but support the use of dopamine agonists in early Parkinson's disease, as a means of delaying L-DOPA therapy and reducing the risk of developing dyskinesia.

Dopamine-receptor stimulation: biobehavioral and biochemical consequences

The MPTP monkey is a well-characterized animal model of parkinsonism and provides an exceptional tool for the study of dyskinesias induced by dopamine-like agents. Several such agents have been tested during the past 15 years, and it has been found that the duration of action of these compounds is the most reliable variable with which to predict their dyskinesiogenic profile. It is proposed that L-dopa-induced dyskinesias represent a form of pathological learning caused by chronic pulsatile (nonphysiological) stimulation of dopamine receptors, which activates a cascade of molecular and biochemical events. These events include defective regulation of Fos proteins that belong to the deltaFosB family, increased expression of neuropeptides, and defective GABA- and glutamate-mediated neurotransmission in the output structures of the basal ganglia.

Continuous dopamine-receptor stimulation in early Parkinson's disease

Chronic L-dopa therapy is associated with the development of motor complications in the majority of Parkinson's disease (PD) patients. Although the precise mechanism responsible for these events is not known, increasing laboratory and clinical evidence points to a sequence of events that is initiated by abnormal pulsatile stimulation of dopamine receptors by the intermittent administration of agents with short half-lives such as L-dopa. Initiating therapy with a long-acting dopamine agonist has been shown to delay the onset and reduce the severity of motor complications in MPTP monkeys and PD patients. Administering L-dopa with a catechol-O-methyltransferase (COMT) inhibitor to block its peripheral metabolism increases its plasma half-life and might have a similar effect. Thus, a rational strategy for treating PD would be to initiate therapy with a long-acting dopamine-receptor agonist and supplement at the appropriate time with L-dopa combined with a COMT inhibitor.

Temporal changes of dopaminergic and glutamatergic receptors in 6-hydroxydopamine-treated rat brain

Quantitative receptor autoradiography was used to examine the sequential patterns of changes in dopaminergic and glutamatergic receptors in the brain of rats lesioned with 6-hydroxydopamine. The animals were unilaterally lesioned in the medial forebrain bundle and the brains were analyzed at 1, 2, 4 and 8 weeks of postlesion. Degeneration of the nigrostriatal pathway caused a significant increase in dopamine D(2) receptors in the ipsilateral striatum from 1 to 8 weeks of postlesion. In the ipsilateral substantia nigra (SN), a significant decrease in dopamine D(2) receptors was also observed from 1 to 8 weeks of postlesion. On the other hand, dopamine D(1) receptors were increased in the ipsilateral ventromedial striatum from 2 to 4 weeks of postlesion. In the ipsilateral SN, a transient increase in dopamine D(1) receptors was observed only 1 week after lesioning. However, other regions in both ipsilateral and contralateral sides showed no significant change in dopamine D(1) and D(2) receptors during postlesion except for a transient change in a few regions. N-Methyl-D-aspartate (NMDA) receptors showed no significant changes in all brain regions studied during the postlesion. In contrast, a transient increase in excitatory amino acid transport sites was observed only in the frontal cortex and ventromedial striatum of the ipsilateral side at 2 weeks of postlesion. However, glycine receptors showed a significant change in any brain areas of both ipsilateral and contralateral sides after lesioning. The change in the brain areas of contralateral side was more pronounced than that of ipsilateral side for glycine receptors. In addition, dopamine uptake sites showed a severe damage in the ipsilateral striatum from 1 to 8 weeks after lesioning. In the contralateral side, in contrast, no significant change in dopamine uptake sites was found in the striatum during the postlesion. These results indicate that unilateral injection of 6-hydroxydopamine in the medial forebrain bundle can cause a significant increase in dopamine D(1) and D(2) receptors in the striatum. The increase in dopamine D(2) receptors was more pronounced than that in dopamine D(1) receptors in the striatum after 6-hydroxydopamine treatment. In contrast, dopamine uptake sites showed a severe damage in the striatum during the postlesion. Furthermore, our results support the existence of dopamine D(2) receptors on the neurons of SN, but not dopamine D(1) receptors. For glutamatergic receptor system, the present study suggests that the changes in glycine receptors may be more susceptible to degeneration of nigrostriatal pathway than NMDA receptors and excitatory amino acid transport sites. Thus, our findings are of interest in relation of degeneration of the nigrostriatal pathway that occurs in Parkinson's disease

Effects of oligonucleotide antisense to dopamine D(1A) receptor messenger RNA in a rodent model of levodopa-induced dyskinesia

Dyskinesias are abnormal involuntary movements which develop as a side-effect of long-term treatment with levodopa in patients with Parkinson's disease. The pathophysiology underlying these dyskinesias remains unclear, although, it has been suggested that heightened activity of dopamine D(1) receptor-bearing striatonigral neurons may play a key role. Chronic pulsatile levodopa administration to hemiparkinsonian rats results in sensitization of rotational responses to apomorphine. This sensitization is thought to be analogous to levodopa-induced dyskinesias in humans. In these studies, we further clarify the role of the dopamine D(1A) receptor in this rodent model of levodopa-induced dyskinesias using an in vivo oligonucleotide antisense approach. Hemiparkinsonian rats received twice daily injections of levodopa for three weeks followed by intrastriatal infusion of dopamine D(1A) receptor antisense (7nmol/day, three days), a scrambled missense control sequence, or saline. Those animals treated with antisense displayed significantly fewer apomorphine-induced rotations than saline- or missense-treated controls.By reducing dopamine D(1A) receptor expression, we were able to attenuate sensitization of the response to apomorphine resulting from chronic pulsatile levodopa treatment. Thus, the dopamine D(1A) receptor appears to play a significant role in levodopa-induced dyskinesias and warrants further examination. These findings may have important implications for the development of selective treatment strategies designed to alleviate parkinsonian symptoms, while minimizing motor complications.

Functional MRI of basal ganglia responsiveness to levodopa in parkinsonian rhesus monkeys

Functional MRI (fMRI) was used to study striatal sensitivity to levodopa in hemiparkinsonian rhesus monkeys. Responses consistent with increased neuronal activity were seen in areas whose normal dopaminergic input from the substantia nigra pars compacta had been ablated by MPTP. Sites of increased activity following levodopa included the lateral putamen, the ventral region of the caudate head, septal areas, and midlateral amygdala in the MPTP-lesioned hemisphere. Increased activity was also observed in the same areas in the nonlesioned hemisphere, but was less pronounced in spatial extent and magnitude, suggesting either subclinical contralateral damage and/or functional adaptations in the contralateral dopamine systems. The increases in neuronal activity following levodopa treatment were temporally correlated with increases in striatal dopamine levels. Chronic levodopa treatment reduced behavioral responsiveness to levodopa and abolished the fMRI response. These results suggest that fMRI can detect changes in dopamine receptor-mediated neuronal sensitivity to dopaminergic agents.

ABT-431, a D1 receptor agonist prodrug, has efficacy in Parkinson's disease

Studies in animal models show a selective D1 receptor agonist with full functional efficacy compared with dopamine to have antiparkinsonian efficacy of similar magnitude to levodopa, without the same propensity for inducing dyskinesia. To date, no such agent has been tested in humans. ABT-431 is the prodrug of A-86929, a full, selective D1 receptor agonist. Subjects (n = 14) with levodopa-responsive Parkinson's disease received five doses of ABT-431 (5, 10, 20, 30, and 40 mg) and one of placebo after a 12-hour levodopa holiday. Response was assessed by using the Unified Parkinson's Disease Rating Scale motor subsection. Dyskinesia was separately graded. ABT-431 showed efficacy significantly superior to placebo at doses of 10 mg and more, and of similar magnitude to that seen with levodopa. Dyskinesia was reduced in several patients after receiving ABT-431. There were no serious adverse events, the most common minor events being nausea and emesis, dizziness, and hypotension. Assuming that ABT-431 is not transformed in humans into an unknown active D2 metabolite, and remains selective for D1 receptors, it is the first dopamine D1 receptor agonist to demonstrate a full antiparkinsonian effect in patients with Parkinson's disease. These preliminary findings also suggest that it may exhibit a reduced tendency to provoke dyskinesia. The emergence of a well-tolerated D1 agonist should allow for the development of a better understanding of the relation between motor efficacy and dyskinesia in Parkinson's disease.

Experimental models of Parkinson's disease: from the static to the dynamic

The experimental models of Parkinson's disease (PD) available today can be divided into two categories according to the mode of action of the compound used: transient pharmacological impairment of dopaminergic transmission along the nigrostriatal pathway or selective destruction by a neurotoxic agent of the dopaminergic neurons of the substantia nigra pars compacta. The present article looks at the relative merits of each model, the clinical symptoms and neuronal impairment it induces, and the contribution it could make to the development of a truly dynamic model. It is becoming more and more clear that there is an urgent need for a chronic model integrating all the clinical features of PD including resting tremor, and reproducing the gradual but continuous nigral degeneration observed in the human pathology. Discrepancies have been reported several times between results obtained in classic animal models and those described in PD, and it would seem probable that such contradictions can be ascribed to the fact that animal models do not, as yet, reproduce the continuous evolution of the human disease. Dynamic experimental models which come closer to the progressive neurodegeneration and gradual intensification of motor disability so characteristic of human PD will enable us to investigate crucial aspects of the disease, such as compensatory mechanisms and dyskinesia.

Upregulation of striatal D2 receptors in the MPTP-treated vervet monkey is reversed by grafts of fetal ventral mesencephalon: an autoradiographic study

Although neural transplantation holds promise as a treatment for Parkinson's disease, parkinsonian primates have generally exhibited inconsistent and incomplete recovery of motor functions following intrastriatal grafting of fetal ventral mesencephalon. One possible contributing factor to this variable response is lack of appropriate integration of donor neurons with host striatal circuitry with the result that there is insufficient dopamine release and postsynaptic dopamine receptor activation. This issue was examined by measuring the effect of transplanting fetal ventral mesencephalon to the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated (MPTP) monkeys on striatal D2 receptor binding. One year after receiving MPTP, D2 receptor binding was upregulated in the dorsal and ventral striatum of African green monkeys. Grafting of fetal ventral mesencephalon to the dorsal striatum of MPTP-treated monkeys 9 months before sacrifice, eliminated the D2 receptor upregulation in dorsal, but not ventral, region. Dopamine concentration in dorsal striatum of grafted MPTP-treated monkeys was significantly higher than in that region of MPTP-treated non-grafted monkeys. In addition, dopamine concentration was significantly higher in dorsal compared to ventral striatum of grafted MPTP-treated monkeys. These data, in addition to those from a previous autoradiographic study on dopamine uptake site density in these monkeys, strongly supports the hypothesis that ectopically placed ventral mesencephalon not only produces, but maintains the release of sufficient levels of dopamine to restore postsynaptic dopamine transmission in regions influenced by graft-derived dopamine.

Continuous or pulsatile chronic D2 dopamine receptor agonist (U91356A) treatment of drug-naive 4-phenyl-1,2,3,6-tetrahydropyridine monkeys differentially regulates brain D1 and D2 receptor expression: in situ hybridization histochemical analysis

The effect of a chronic D2 dopamine receptor agonist (U91356A) treatment on dopamine receptor gene expression in the brain of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys was investigated using quantitative in situ hybridization histochemistry. U91356A was administered to MPTP-monkeys for 27 days in a pulsatile (n=3) or continuous (n=3) schedule. Animals treated in a pulsatile mode showed progressive sensitization and developed dyskinesia; whereas with the continuous mode behavioural tolerance was observed but no dyskinesia developed. Untreated MPTP as well as naive control animals were also studied. The efficacy and uniformity of the MPTP effect was assessed by measures of dopamine concentrations by high performance liquid chromatography with electrochemical detection in the relevant brain areas. D1 and D2 receptor messenger RNAs levels were examined by in situ hybridization histochemistry using human complementary RNA probes. Intense specific labelling for D1 and D2 receptor messenger RNAs was measured in the caudate and putamen with a rostrocaudal gradient for D2 receptors and a lower density in the cortex for D1 receptors messenger RNA. D1 receptor mRNA levels in rostral striatum and cortex decreased whereas D2 receptor messenger RNA in caudal striatum increased in MPTP-monkeys compared to control animals. Continuous administration of U91356A reversed the MPTP-induced increase of D2 receptor messenger RNA, whereas the pulsatile administration did not significantly correct these messenger RNA changes. U91356A treatment whether continuous or pulsatile partially corrected the D1 receptor messenger RNA lesion-induced decrease in the striatum, whereas no correction was observed in the cortex. All MPTP-monkeys were extensively and similarly denervated suggesting that the D1 and D2 receptor expression changes following U91356A administration were treatment related. Our data show a lesion-induced imbalance of D1 (decrease) and D2 (increase) receptor messenger RNAs in the striatum of MPTP-monkeys. The response of these receptors to D1 agonist treatment showed receptor selectivity and was influenced by the time-course of drug delivery. Hence chronic continuous but not pulsatile administration of U91356A reversed the striatal D1 receptor messenger RNA increase.

Cabergoline, a long-acting dopamine D2-like receptor agonist, produces a sustained antiparkinsonian effect with transient dyskinesias in parkinsonian drug-naive primates

Continuous dopaminergic receptor stimulation is now considered as an interesting approach for the control of motor complications often seen in parkinsonian patients treated chronically with levodopa. Cabergoline, which is a long-acting dopamine D2-like receptor agonist, has been tried recently with good results as an adjunct in patients already on levodopa-therapy. Thus, the present study was designed to test the effects of repeated s.c. administration of cabergoline as sole therapeutic agent during a month in 3 drug-naive MPTP parkinsonian monkeys to see whether or not cabergoline, given every other day at 0.25 mg/kg, would have a sustained antiparkinsonian effect and would induce dyskinesias. The animals were rated to quantify the antiparkinsonian as well as the dyskinetic response and gross locomotor activity was monitored by photocells. The averaged locomotor response, initially greatly increased (approximately 9 times higher than after saline treatment in the same animals), decreased by approximately 50% after 2 weeks but was thereafter maintained at this level until the end of the study. The parkinsonian features were improved in a sustained manner in all monkeys and transient dyskinesias (week 1 and 2) were present in 2 of 3 monkeys. After sacrifice receptor binding assays were performed on striatal and pallidal tissues homogenates with tritiated selective ligands and compared with those of 3 normal and 3 MPTP-exposed monkeys otherwise untreated. A significant decrease in dopamine D2-like receptor density in the putamen (-36% on average vs. untreated MPTP-exposed monkeys) may be involved in the behavioral partial tolerance to antiparkinsonian effect of cabergoline and the disappearance of dyskinesias. A reversal of the supersensitivity of GABAA receptor in the internal segment of the globus pallidus (-15% on average vs. untreated MPTP-exposed monkeys) may also be implicated in this latter behavioral effect.

Dyskinesias and tolerance induced by chronic treatment with a D1 agonist administered in pulsatile or continuous mode do not correlate with changes of putaminal D1 receptors in drug-naive MPTP monkeys

Nine monkeys (Macaca fascicularis) were rendered parkinsonian after intravenous administration of the toxin MPTP. Three of these animals received pulsatile administration of the D1 receptor agonist SKF 82958 (1 mg/kg, three times daily) while three were treated by continuous infusion via an osmotic mini-pump with SKF 82958 (at an equivalent amount daily) for 29 days. Untreated MPTP as well as healthy control animals were also studied. Relief of parkinsonian symptoms was observed in the three animals of the pulsatile group. However, dyskinesia occurred in two monkeys which had striatal dopamine depletion of > 99% compared to the non-dyskinetic animal slightly less denervated (94%). Monkeys receiving continuous SKF 82958 showed no anti-parkinsonian effect and no dyskinesia. All monkeys from the pulsatile and continuous group had measurable amount of plasma SKF 82958 as assayed by HPLC with electrochemical detection. In the putamen of all SKF 82958-treated monkeys, Bmax of D1 receptors labeled with [3H]SCH 23390 were increased versus untreated MPTP-monkeys with no change in Kd. In contrast, a decrease D1 receptor density was observed in the nucleus accumbens of untreated MPTP monkeys versus controls and this was not corrected with either pulsatile or continuous SKF 82958 treatments. D2 receptor density measured with [3H]spiperone binding was increased in the posterior putamen of SKF 82958-treated monkeys whereas no change was observed in the accumbens compared to control animals. Hence, tolerance with the continuous administration of a D1 agonist is not associated with a decrease of putaminal D1 or D2 receptor densities and dyskinesia could not be specifically associated with an increase of putaminal D1 receptors.

Dopamine D3 receptors in the basal ganglia of the common marmoset and following MPTP and L-DOPA treatment

The distribution of the dopamine D3 receptor was studied by receptor autoradiography using [3H]7-OH-DPAT in striatal and extrastriatal brain regions of the common marmoset (Callithrix jacchus). Saturation studies demonstrated that [3H]7-OH-DPAT bound with similar affinity to different regions of marmoset brain. In normal marmosets, specific [3H]7-OH-DPAT binding was found in both striatal and extrastriatal regions. Very high levels of specific [3H]7-OH-DPAT binding were detected in the islands of Calleja and nucleus accumbens but in addition high levels of binding were detected in rostral caudate nucleus and putamen. In common marmosets treated with the selective nigral neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), the levels of specific [3H]7-OH-DPAT binding in striatal and extrastriatal regions were not different to those in normal animals. Chronic treatment of MPTP-treated marmosets with L-DOPA/ carbidopa did not alter the levels of specific [3H]7-OH-DPAT binding in any brain region. These results demonstrate that in common marmosets D3 receptors are located in both striatal and limbic regions. The receptor density is not altered by dopaminergic denervation or by chronic L-DOPA administration. The D3 receptor may, therefore, be important in both the therapeutic and adverse effects of drugs used to treat Parkinson's disease.

Chronic L-DOPA administration induces dyskinesias in the 1-methyl-4- phenyl-1,2,3,6-tetrahydropyridine-treated common marmoset (Callithrix Jacchus)

Dyskinesias occur in the majority of patients with Parkinson's disease chronically treated with L-DOPA and also occur in several nonhuman primate species after 1-methyl-4phenyl-1,2,3,6-tetrahydropyridine (MPTP) and L-DOPA treatment. The common marmoset (Callithrix jacchus) shows parkinsonian motor deficits after MPTP administration, and we now report dyskinesias occurring in this species during chronic L-DOPA exposure. Marmosets rendered chronically parkinsonian after MPTP administration were treated orally with L-DOPA plus carbidopa for 3 weeks. After several days the animals began to display chorea, choreoathetosis, and dystonia. The severity of dyskinesias varied between the animals, with the most severely parkinsonian animals displaying the most dyskinetic movements. Each animal showed an idiosyncratic pattern of dyskinesias, which was highly reproducible. These L-DOPA-primed animals also received other D2 D1, and mixed D1/D2 agonist drugs. Quinpirole, bromocriptine, pergolide, apomorphine, and A-77636 all produce dyskinesias that were identical in character to those seen after L-DOPA administration, but the D1 agonist A-77636 gradually abolished dyskinesias while preserving its antiparkinsonian activity. The MPTP-treated marmoset provides a useful model in which to study dyskinesias in Parkinson's disease and to examine new therapeutic strategies aimed at alleviating this common side effect of chronic dopamine replacement therapy.

Is striatal dopaminergic receptor imbalance responsible for levodopa-induced dyskinesia?

Abnormal involuntary movements (dyskinesias) of variable intensity eventually emerge in the majority of Parkinson's disease patients chronically treated with standard oral levodopa. They create social and physical embarrassment and narrow the therapeutic options normally proposed to improve Parkinsonian symptoms. Thus far, indirect clinical and experimental evidence has implicated the potential role of dopamine D1 receptor activation in the generation of dopa dyskinesia. In recent years, our group has tested several dopaminergic agonists of variable half-life and selectivity in monkeys rendered parkinsonian following toxic exposure to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). These monkeys readily develop dyskinesia when treated with levodopa and provide the best animal model to study this complication. Our results in "drug-naive" and "dyskinesia-primed" MPTP animals suggest that pathological sensitisation of D2 receptor-mediated striatal outflow is necessary and sufficient for the induction of dopa dyskinesia, with perhaps a synergistic contribution from D1 receptors, and that repeated short-lived stimulation is important in the sensitisation process. This model supports the hypothesis that more continuous forms of dopaminomimetic therapy represent the best therapeutic approach for Parkinson's disease and calls for the development of novel D1 agonists for further clinical testing.

L-DOPA reverses altered gene expression of substance P but not enkephalin in the caudate-putamen of common marmosets treated with MPTP

The mRNA levels encoding neuropeptides were measured in the caudate nucleus, putamen and nucleus accumbens of common marmosets exposed to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine pyridine (MPTP). Motor deficits induced by MPTP treatment were characterized by akinesia, postural abnormalities and rigidity. Seven days after MPTP treatment, there was a marked increase in levels of enkephalin mRNA in the caudate nucleus and putamen. In contrast, the hybridization signal for substance P mRNA was reduced. Alterations in the mRNA encoding neuropeptides were similar but less extensive in marmosets at 18-50 months following MPTP treatment. No significant changes in enkephalin or substance P mRNA in the nucleus accumbens were observed at either time. Treatment with L-DOPA plus carbidopa for 4 weeks reversed MPTP-induce motor deficits and other behavioural abnormalities. The decrease in substance P mRNA in the striatum of MPTP-treated animals was reversed by L-DOPA treatment and reached levels above those found in normal animals. In contrast, the increase in enkephalin mRNA in marmosets treated with MPTP was not altered by L-DOPA treatment. In the nucleus accumbens the levels of peptide mRNA were not affected by L-DOPA treatment. Loss of nigral dopamine cells in a primate species causes opposing alterations in the expression of enkephalin and substance P mRNA in the caudate nucleus and putamen. No changes were observed in the nucleus accumbens, which reflects the resistance of the mesolimbic neurons to MPTP toxicity. While the decrease in substance P mRNA was reversed by L-DOPA treatment, the increase in enkephalin mRNA was not. This may partly indicate the greater effect of L-DOPA on the direct GABA pathway compared to the indirect output pathway from the striatum.

Effect of adding the D-1 agonist CY 208-243 to chronic bromocriptine treatment of MPTP-monkeys: regional changes of brain dopamine receptors

1. Eleven monkeys were administered N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP): eight were treated with bromocriptine for one week and then CY 208-243 (four monkeys) or saline (four monkeys) was added to the bromocriptine treatment. 2. Addition of CY 208-243 increased the therapeutic response observed with the ergot alone without inducing dyskinesia. 3. Following MPTP, [3H]-SCH 23390 specific binding to D-1 receptors as well as [3H]-spiperone and [3H]-N-n-propylnorapomorphine specific binding to D-2 receptors increased in posterior striatum compared to control animals, whereas [3H]-SKF 38393 binding to D-1 receptors tended to decrease. 4. Dopamine receptor density was unchanged in anterior striatum of untreated MPTP-monkeys. 5. In the posterior striatum, both dopaminergic treatments decreased towards control values [3H]-SCH 23390, [3H]-spiperone and [3H]-N-n-propylnorapomorphine binding whereas they did not significantly change [3H]-SKF 38393 specific binding. [3H]-SKF 38393 specific binding increased in anterior striatum of bromocriptine-treated MPTP-monkeys, compared to untreated MPTP-animals, and this increase was abolished in animals treated with bromocriptine+CY 208-243. 6. The present study shows that in MPTP-monkeys, treated or not with DA agonists, the D1 and D2 receptor changes are concentrated in the posterior striatum and that denervation appears to cause a shift from the high to the low affinity agonist state of D1 receptors but not for the D2 subtype.

Levodopa or D2 agonist induced dyskinesia in MPTP monkeys: correlation with changes in dopamine and GABAA receptors in the striatopallidal complex

Dopamine D1 and D2 receptors as well as the GABA/benzodiazepine receptor complex in the striatum and the globus pallidus (internal: GPi and external: GPe) were studied by autoradiography using [3H]SCH 23390, [3H]spiperone, and [3H]flunitrazepam ([3H]FNZ) respectively, in five groups of cynomolgus monkeys. These included (i) untreated 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-monkeys; (ii) MPTP monkeys treated chronically with levodopa injections; (iii) MPTP monkeys treated chronically with injections of the novel D2 agonist U91356A; (iv) MPTP monkeys treated chronically with U91356A delivered through an osmotic mini-pump; and (5) naive controls. Animals treated in a pulsatile mode with U91356A or levodopa injections showed progressive sensitization to their respective drug and developed choreic dyskinesia. In contrast, animals treated in a continuous mode with U91356A showed behavioral tolerance but did not develop dyskinesia. A trend for a down-regulation of putaminal D2 receptors was observed following D2 agonist stimulation with U913356A. Striatal [3H]FNZ binding was significantly decreased only in animals treated in a continuous mode with U91356A. The dopamine receptor decrease in the striatum could be implicated with the development of tolerance but cannot explain the appearance of dyskinesia. Denervation by MPTP was associated with a decrease of the GPe/GPi [3H]FNZ binding ratio which reflects an imbalance of striatal output pathways; this ratio was not reversed by any of the treatments although changes were observed in the GPe and GPi. Indeed, pulsatile U91356A treatment restored the decreased [3H]FNZ binding in the GPe near control values and levodopa showed a similar tendency. A significant increase of [3H]FNZ binding in the GPi only of dyskinetic monkeys, namely those treated with pulsatile U91356A or levodopa was seen compared to untreated MPTP or naive controls. This GABAA receptor up-regulation might lead to a supersensitive state of the GPi to gabaergic input which may be involved in the mechanism underlying the development of dopaminomimetic-induced dyskinesia.

Alterations in dopamine uptake sites and D1 and D2 receptors in cats symptomatic for and recovered from experimental parkinsonism

The administration of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to adult cats severely disrupts the dopaminergic innervation of the striatum. Animals display a parkinson-like syndrome, consisting of akinesia, bradykinesia, postural instability, and rigidity, which spontaneously recovers by 4-6 weeks after the last administration of MPTP. In this study we used quantitative receptor autoradiography to examine changes in DA uptake sites and DA receptors in the basal ganglia of normal, and symptomatic and recovered MPTP-treated cats. Consistent with the destruction of the nigrostriatal DA pathway, there was a severe loss of DA uptake sites, labeled with [3H]-mazindol, in the caudate nucleus (64-82%), nucleus accumbens (44%), putamen (63%), and substantia nigra pars compacta (SNc, 53%) of symptomatic cats. Following behavioral recovery, there were no significant changes in DA uptake site density. Significant increases of [3H]-SCH 23390 binding to D1 DA receptors were observed in the dorsal caudate (> 24%; P < 0.05) of symptomatic cats and in all regions of the caudate-putamen (> 30%; P < 0.05) of recovered animals. [3H]-SCH 23390 binding in the substantia nigra pars reticulata was half of that in the striatum and showed no changes in symptomatic or recovered animals. No alterations in the binding of [125I]-epidepride to D2 receptors was observed in any region of the striatum in either symptomatic or recovered animals. [125I]-Epidepride binding in the SNc was decreased by > 36% (P < 0.05) following MPTP treatment. These data show that cats made parkinsonian by MPTP exposure have a significant decrease in the number of DA reuptake sites throughout the striatum and that recovery of sensorimotor function in these animals is not correlated with an increase in the number of striatal reuptake sites. Behavioral recovery, however, does seem to be correlated with a general elevation of D1 receptors throughout the striatal complex. The present data also show that direct correlations between changes in DA receptor regulation after a large DA depleting lesion and behavioral deficits or recovery from those deficits are difficult and that the relationships between DA receptors/transporters and behavior require further study.

Loss of D1/D2 dopamine receptor synergisms following repeated administration of D1 or D2 receptor selective antagonists: electrophysiological and behavioral studies

Many effects resulting from D2 dopamine (DA) receptor stimulation are manifest only when D1 DA receptors are stimulated by endogenous DA. When D1 receptor stimulation is enhanced by administration of selective D1 receptor agonists, the functional effects of selective D2 agonists are markedly increased. These qualitative and quantitative forms of D1/D2 DA receptor synergism are abolished by chronic DA depletion when both D1 and D2 DA receptors are supersensitive. Using both electrophysiological and behavioral methods, the present study examined the effects of selective D1 and D2 receptor supersensitivity, induced by repeated administration of selective D1 or D2 receptor antagonists, on the synergistic relationships between D1 and D2 receptors. Daily administration of the selective D2 antagonist eticlopride (0.5 mg/kg, s.c.) for 3 weeks produced a selective supersensitivity of both dorsal (caudate-putamen) and ventral (nucleus accumbens) striatal neurons to the inhibitory effects of the D2 agonist quinpirole (applied by microiontophoresis). This treatment also abolished the normal ability of the D1 agonist SKF 38393 to potentiate quinpirole-induced inhibition, and relieved D2 receptors from the necessity of D1 receptor stimulation by endogenous DA (enabling), as indicated by significant electrophysiological and behavioral (stereotypy) effects of quinpirole in eticlopride-pretreated, but not saline-pretreated, rats that were also acutely depleted of DA. Daily administration of the selective D1 receptor antagonist SCH 23390 (0.5 mg/kg, s.c.) caused supersensitivity of striatal neurons to the inhibitory effects of SKF 38393 and also abolished both the ability of SKF 38393 to potentiate quinpirole-induced inhibition and the necessity of D1 receptor stimulation for such inhibition. However, both quinpirole-induced inhibition of striatal cells and stereotyped responses were also somewhat enhanced in SCH 23390-pretreated rats. When such D1-sensitized rats were acutely depleted of DA, the behavioral effects of quinpirole were intermediate between saline-pretreated rats with acute DA depletion and SCH 23390-pretreated rats without acute DA depletion. Based upon these and related results, it is argued that the enhanced effects of quinpirole in D1-sensitized rats are due to a heterologous sensitization of D2 receptors rather than to enhanced enabling resulting from supersensitive D1 receptors. It is suggested that supersensitivity of either D1 or D2 receptors can lead to an uncoupling of normal qualitative and quantitative D1/D2 synergisms and that the heterologous regulation of D2 receptor sensitivity by D1 receptors may be related to uncoupling of functional D1/D2 synergisms.

Chronic CY 208-243 treatment of MPTP-monkeys causes regional changes of dopamine and GABAA receptors

Four monkeys were rendered parkinsonian by N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) i.v. administration and then treated chronically with increasing doses of the D1 agonist CY 208-243 (0.05, 0.1 and 0.5 mg/kg). All animals showed a dose-dependent improvement of their parkinsonian signs after the chronic CY 208-243 treatment; however, half of them developed peak-dose dyskinesias. Dopamine levels were more decreased in the striatum of MPTP-monkeys with dyskinesias compared to those without dyskinesias. [3H]SCH 23390 and [3H]SKF 38393 binding to D1 receptors were in general similar in the striatum of both groups of MPTP-monkeys except [3H]SKF 38393 binding which was lower in the posterior putamen of dyskinetic compared to non-dyskinetic monkeys reflecting decreased coupling of this receptor to G proteins. [3H]spiperone and [3H]N-n-propylnorapomorphine binding to D2 receptors in the striatum tended in general to be higher in dyskinetic compared to non-dyskinetic monkeys, and this reached statistical significance in the posterior caudate labelled with [3H]n-propylnorapomorphine. [3H]muscimol binding to GABAA receptors was significantly higher in the posterior caudate of dyskinetic compared to non-dyskinetic monkeys. The extent of striatal DA denervation, decreased D1, elevated D2 and GABAA receptors, as well as the decrease of the D1/D2 receptor ratio in the posterior striatum may be involved in the appearance of dyskinesias after chronic CY 208-243 treatment.

Loss of striatal cholinergic neurons as a basis for tardive and L-dopa-induced dyskinesias, neuroleptic-induced supersensitivity psychosis and refractory schizophrenia

In the first section of this paper several aspects of tardive dyskinesia (TD) (clinical, epidemiological, pharmacological) are reviewed. We propose that this syndrome is not the consequence of dopamine receptor proliferation, but results from damage or degeneration of striatal cholinergic interneurons. We suggest that this cellular damage is caused by prolonged overactivation of these neurons, which occurs when they are released from dopaminergic inhibition following neuroleptic administration. Overactivity of central cholinergic systems during akinetic and motor retarded depression could be a contributory cause. The predisposition to L-DOPA-induced peak-dose dyskinesia in Parkinson's disease may depend on the same type of striatal neuronal loss. In the second part of the paper, the subject of supersensitivity psychosis and drug-resistant schizophrenia is reviewed. These two syndromes, are commonly associated with TD, have similar predisposing factors and pharmacology to TD, and are potentially persistent. We suggest that these conditions also result from degeneration of cholinergic striatal interneurons following chronic neuroleptic administration. The efficacy of clozapine for such treatment-refractory psychoses is explained in terms of its blockade of D-1 dopamine receptors. Other drugs effective against refractory psychoses (e.g. risperidone) are predicted to reduce activation at D-1 receptors.

Alterations in striatal and extrastriatal D-1 and D-2 dopamine receptors in the MPTP-treated common marmoset: an autoradiographic study

In adult common marmosets (Callithrix jacchus), MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) treatment induced almost total depletion of cells in the substantia nigra pars compacts (SNc) but partial cell loss in the ventral tegmental area (VTA). There was severe depletion of [3H]-mazindol binding to dopamine (DA) uptake sites in the caudate, putamen, and SNc. The loss of [3H]-mazindol binding in the nucleus accumbens (NAc) and olfactory tubercle (OT) was less marked. [3H]-mazindol binding in the body of caudate nucleus showed a small but significant recovery with increasing post-lesion survival times. The specific binding of [3H]-SCH 23390 to D-1 DA receptor sites was increased after MPTP treatment in all subregions of both caudate and putamen but was unaltered in the NAc and OT. Substantia nigra pars reticulata (SNr), frontal cortex, and medial segment of globus pallidus (GPm) all demonstrated moderate levels of [3H]-SCH 23390 binding in control animals, which were unaffected by MPTP treatment. Specific [3H]-spiperone binding to D-2 DA receptor sites was not altered by MPTP treatment in the subregions of caudate-putamen. Moderate levels of [3H]-spiperone binding were observed in control animals in the NAc, OT, SNc, and the lateral segment of globus pallidus (GP1). [3H]-spiperone binding in the SNc and OT was partially decreased in MPTP-treated animals. The changes in specific [3H]-spiperone and [3H]-SCH 23390 binding induced by MPTP-treatment did not alter with post-lesion survival times. These results demonstrate that MPTP treatment causes greater dopaminergic denervation of the caudate-putamen than in NAc/OT. This resulted in an increase in postsynaptic D-1 DA receptor sites in the caudate-putamen but not in the NAc/OT. Also, there appeared to be loss of presynaptic D-2 DA receptor sites in the SNc and OT. In the caudate-putamen, the loss of presynaptic D-2 DA receptor sites may have masked postsynaptic D-2 DA receptor upregulation.

The dopamine D1 receptor agonist SKF 38393 suppresses detrusor hyperreflexia in the monkey with parkinsonism induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

A pharmacological study using monkeys, in which parkinsonism was induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), was undertaken to elucidate the mechanism underlying urinary bladder dysfunctions in Parkinson's disease. Under ketamine anesthesia, cystometrograms showed that, in MPTP-treated monkeys, a contraction of the urinary bladder was induced with smaller bladder volume than that in normal monkeys. In MPTP-treated monkeys, subcutaneously injected SKF 38393, a dopamine D1 receptor agonist, significantly increased the bladder volume and pressure thresholds for inducing the micturition reflex without affecting those in normal monkeys. In contrast, subcutaneous injections of quinpirole, a dopamine D2 receptor agonist, and apomorphine, a dopamine D1 and D2 receptor agonist, slightly, but significantly reduced the volume threshold of the bladder for the micturition reflex in both normal and MPTP-treated groups. These results indicate that, in parkinsonism, the degeneration of dopaminergic neurons in the substantia nigra leads to the detrusor hyperreflexia, probably due to a failure of activation of dopamine D1 receptors.

Effect of adding the D1 agonist CY 208-243 to chronic bromocriptine treatment. I: Evaluation of motor parameters in relation to striatal catecholamine content and dopamine receptors

A group of four cynomolgus monkeys previously rendered parkinsonian by the toxin 1-methyl-4-phenyl,1,2,3,6-tetrahydropyridine (MPTP) were observed in locomotion cages equipped with photocells during four periods of 7 days during which they received saline or two doses of the D1 agonist CY 208-243. The larger dose of 0.5 mg/kg produced a significant increase in locomotion in three of four animals. A second group of eight monkeys also previously rendered parkinsonian by MPTP and having received no other treatment were given a daily treatment of bromocriptine 1.66 mg/kg orally daily during 4 weeks. In four of the animals, after a week on bromocriptine alone, the D1 agonist CY 208-243 was added in increasing doses of 0.05, 0.1, and 0.5 mg/kg. The motor response as measured by locomotion, hand dexterity, and a disability score improved progressively at least in some of the animals on bromocriptine alone. The addition of CY 208-243 produced a more striking improvement of all three parameters, which appeared to be dose dependent. Biochemical analysis of the brain of these animals plus one control and one MPTP untreated monkey showed a > 90% loss of dopamine in the striatum in six of the eight treated monkeys. Both D2 and D1 dopamine receptors were increased in density by denervation, but both treatments abolished this increase for the D2 receptors while increasing the affinity of the D1 receptors.

Dopamine D1 receptor stimulation but not dopamine D2 receptor stimulation attenuates haloperidol-induced behavioral supersensitivity and receptor up-regulation

The effect of selective dopamine D1 and D2 receptor agonists on chronic haloperidol-treated rats was studied. Haloperidol treatment produced a 77% increase in apomorphine-induced sterotypy. The administration of the selective dopamine D1 receptor agonist SKF38393 alone or in combination with the selective dopamine D2 receptor agonist quinpirole attenuated the effect of haloperidol. Treatment with quinpirole alone did not have a significant effect on the response to haloperidol. Haloperidol did not modify the number of dopamine D1 receptors but increased that of dopamine D2 receptors. SKF38393 reversed the effect of haloperidol on dopamine D2 receptor binding. Co-administration of SKF38393 and quinpirole did not modify the increase in the number of dopamine D2 receptors induced by chronic treatment haloperidol. The results confirm a dissociation between behavioral supersensitivity and dopamine receptor up-regulation, suggesting that other mechanisms may be involved in the expression of behavioral supersensitivity.

Differential effect of chronic dopaminergic treatment on dopamine D1 and D2 receptors in the monkey brain in MPTP-induced parkinsonism

Dopamine D1 and D2 receptors located within the striatum (caudate nucleus and putamen) were studied autoradiographically, using [3H]SCH 23390 and [3H]sulpiride respectively, in (i) seven monkeys rendered parkinsonian by the systemic administration of MPTP, four of which were chronically exposed to anti-parkinsonian drugs (levodopa or apomorphine), (ii) two hemi-parkinsonian monkeys (induced by intra-carotid infusion of MPTP), one of which received chronic exposure to apomorphine, and (iii) three control monkeys which received neither MPTP nor dopaminergic drugs. Anti-parkinsonian drug exposure resulted in a reversal of symptoms and was accompanied by the development of limb dyskinesias. In parkinsonian monkeys not chronically exposed to drugs. [3H]SCH 23390 binding was slightly but not significantly elevated above control values, whilst in the same animals [3H]sulpiride binding was significantly increased above that found in the control group. Rostrally [3H]SCH 23390 binding was similar in the control and drug-exposed parkinsonian groups but more caudally there was a small consistent, although not significant, increase in [3H]SCH 23390 binding in the drug-exposed animals as compared to the parkinsonian monkeys not exposed to drugs. In contrast at all rostro-caudal levels [3H]sulpiride binding in the drug-exposed parkinsonian group was lower than the corresponding values from the non-drug exposed animals. [3H]SCH 23390 binding showed no major side-to-side difference in the hemi-parkinsonian animal which was not exposed to levodopa/apomorphine, whilst in the hemi-parkinsonian monkey which received apomorphine there was again an increase in binding on the MPTP-treated side of the brain. In both drug- and non-drug exposed hemi-parkinsonian animals there was a greater density of [3H]sulpiride binding in the parkinsonian side of the brain; the general level of binding in the drug-exposed monkey was less than that seen in the other animal. These results would support the idea that in MPTP-induced parkinsonism, dopaminergic denervation results in a greater change in the D2 receptors, but furthermore would indicate a differential effect of levodopa/apomorphine exposure on the D1 and D2 receptor populations. Drug exposure apparently encourages the reversal of the MPTP-induced increase in the D2 receptor binding, whilst the D1 receptor binding appears to proliferate in response to these drugs. These results may have important implications in relation to the development of dyskinesias, subsequent to the chronic use of some anti-parkinsonian drug treatments.

Autoradiographic study of striatal D1 and D2 dopamine receptors in 6-OHDA-lesioned rats receiving foetal ventral mesencephalic grafts and chronic treatment with L-dopa and carbidopa

Foetal dopamine cell suspensions or sham preparations were implanted into the denervated striatum of rats with a unilateral 6-hydroxy-dopamine (6-OHDA) lesion of the medial forebrain bundle. Some animals were also treated with L-DOPA (200 mg/kg/24 h) and carbidopa (25 mg/kg/24 h) in the drinking water for 5 weeks, followed by a 3-week drug-free period. Rotational responses to apomorphine and (+)-amphetamine were assessed, and the density of D1 and D2 dopamine receptors was evaluated autoradiographically in striatal slices exposed to [3H]SCH 23390 or [3H]spiperone. Foetal grafts reduces apomorphine-induced contralateral rotation and prevented the development of apomorphine-induced stereotypy. Foetal grafts abolished (+)-amphetamine-induced ipsilateral rotation. These effects of the grafts were not altered by treatment with L-DOPA. A unilateral 6-OHDA lesion of the nigrostriatal pathway resulted in an ipsilateral increase in D2 receptor density most marked in the lateral and dorsomedial quadrants of the striatum compared with the contralateral side. Foetal ventral mesencephalic grafts implanted into the lesioned striatum decreased D2 receptor density to levels found in the contralateral intact striatum. Chronic L-DOPA and carbidopa treatment did not alter the effect of the grafts. A 6-OHDA lesion resulted in a reduction of D1 receptor density in the lateral areas of the lesioned striatum at Level 2. The presence of a foetal ventral mesencephalic graft either alone or together with L-DOPA treatment did not alter the lesion-induced changes in D1 binding density.

Repeated D1 dopamine receptor agonist administration prevents the development of both D1 and D2 striatal receptor supersensitivity following denervation

Following 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal dopamine (DA) pathway, rat caudate-putamen (CPu) neurons are supersensitive to the inhibitory effects of both D1 and D2 dopamine (DA) receptor selective agonists. In addition, both the necessity of D1 receptor stimulation for D2 agonist-induced inhibition and the synergistic inhibitory effects of D1 and D2 agonists are abolished by denervation. The present study attempted to determine the relative roles of D1 and D2 DA receptors in the development of denervation supersensitivity to DA agonists and the "uncoupling" of functional interactions between the receptors following 6-OHDA lesions of the nigrostriatal DA pathway. Beginning on the day after an intraventricular 6-OHDA (or vehicle) injection, groups of rats received daily injections of either the selective D1 receptor agonist SKF 38393 (8.0 mg/kg, s.c.), the D2 agonist quinpirole (0.5 mg/kg, s.c.), or saline for 7 days. On the day following the last agonist injection, rats were anesthetized and prepared for extracellular single cell recording with iontophoretic drug administration. Daily administration of quinpirole selectively prevented the development of D2 receptor supersensitivity, whereas daily administration of SKF 38393 prevented the development of both D1 and D2 receptor supersensitivity. In addition, D1, but not D2, agonist treatment prevented the loss of synergistic inhibitory responses typically produced by 6-OHDA lesions. Behavioral observations revealed similar effects; daily injections of SKF 38393, but not quinpirole, prevented contralateral rotational responses to the mixed D1/D2 agonist apomorphine (1.0 mg/kg, s.c.) in rats with unilateral 6-OHDA lesions of the nigrostriatal pathway. After a 4-week withdrawal from repeated D1 agonist treatment, both supersensitive inhibitory responses of CPu neurons and contralateral rotations to apomorphine were evident, indicating that the preventative effects on DA receptor supersensitivity were not permanent. These findings indicate that continued agonist occupation of striatal D1 DA receptors following DA denervation not only prevents the development of D1 DA receptor supersensitivity but also exerts a similar regulation of D2 receptor sensitivity.

Effect of estrogen and progesterone on L-dopa induced dyskinesia in MPTP-treated monkeys

A group of 4 cynomolgus monkeys was rendered parkinsonian by the toxin MPTP and then treated daily with L-DOPA until all animals developed dyskinesia. At this point, 17 beta-estradiol, 17 alpha-estradiol and progesterone were injected s.c. singly or in combination for 7 days and the treatment with L-DOPA reinstituted. Our results show that the pretreatment with 17 beta-estradiol reduced the dyskinetic effect without altering the therapeutic response to L-DOPA, while 17 alpha-estradiol did not alter the dyskinetic and the antiparkinsonian effect.

Heterogeneous distribution of dopamine D2 receptor mRNA in the rat striatum: a quantitative analysis with in situ hybridization histochemistry

Dopamine D2 receptor mRNAs have recently been cloned and their gross distribution in the central nervous system described. Quantitative in situ hybridization histochemistry with a cRNA probe complementary to the mRNAs encoding approximately 70% of the third intracellular loop of the rat D2 receptor was performed on sections of rat brain to determine whether differences previously observed in the density of ligand binding sites in subregions of the striatum were related to differences in mRNA levels. Film autoradiographic analysis demonstrated 30% more hybridization signal in the lateral compared to the medial caudate-putamen, a distribution parallel to that of binding of ligands specific for the D2 receptor. Inspection at the cellular level using emulsion autoradiography also indicated a differential distribution of the D2 receptor mRNA. Fewer positively labelled cells, as well as fewer silver grains per cell, were seen in the medial compared to the lateral half of the striatum. This suggests that the gradient seen in autoradiographic studies of the distribution of D2 receptors is related both to regional differences in D2 mRNA levels and to the density of cells expressing the receptor. In addition, the distribution of cells expressing D2 receptor mRNA in the extrastriosomal matrix was compared to that in striosomes identified by the presence of a high density of 3H-naloxone binding sites. Labelled cells were mainly found in the matrix (3H-naloxone binding-poor) but were also seen in striosomes (3H-naloxone binding-rich). The results suggest that differences in levels of D2 binding sites in subregions of the striatum are related to differences in the level of expression of this receptor in intrinsic striatal neurons, suggesting differential regulation of dopamine D2 receptor gene expression in topographically distinct striatal neurons.